Polyaluminoxane compositions that are partial hydrolyzates of alkylaluminums are known to serve as cocatalysts that activate transition metal complexes as the main catalysts in the production of olefin oligomers or olefin polymers. In particular, it is widely known that polymethylaluminoxane compositions prepared from trimethylaluminum as a raw material exhibit excellent cocatalytic performance (Patent Literature 1).
Polymethylaluminoxane compositions are produced by the partial hydrolysis reaction of trimethylaluminum (Patent Literatures 2 and 3) or by the pyrolysis reaction of alkylaluminum compounds which have an aluminum-oxygen-carbon bond formed by the reaction of trimethylaluminum with an oxygen-containing organic compound such as a carboxylic acid (Patent Literatures 4 and 5). Such polymethylaluminoxane compositions are marketed in the form of solutions in aromatic hydrocarbon solvents such as toluene.
When an olefin polymer is produced in such a manner that a solution of the polymethylaluminoxane composition is added as such to the polymerization system as a cocatalyst in the olefin polymerization reaction, it is impossible to control the morphology of the obtainable olefin polymer. Further, stable production is difficult because the process often experiences fouling problems by the deposition of the olefin polymer to apparatuses such as the polymerization reactor.
To realize the stable production of olefin polymers with a good particulate shape, production methods have been disclosed which involve a supported cocatalyst in which a polymethylaluminoxane composition is supported on a solid inorganic carrier such as silica, alumina, silica-alumina or magnesium chloride (Patent Literatures 6 to 9). An advantage in the use of a solid inorganic carrier is that the carrier particle diameter may be selected. In the production of olefin oligomers or olefin polymers, the carrier particle diameter is selected in accordance with the types of processes, namely, whether the process is liquid-phase polymerization such as slurry polymerization or involves a gas-phase polymerization apparatus.
However, these supported cocatalysts which have a polymethylaluminoxane composition supported on a solid inorganic carrier exhibit a markedly lower cocatalytic activity than when the polymethylaluminoxane composition is used alone, thus causing economic disadvantages. Further, the solid inorganic carriers tend to remain as foreign matters in polymers obtained and deteriorate polymer properties.
To solve the above problems, approaches have been proposed in which polyaluminoxane compositions are obtained as solids so that the polyaluminoxane compositions themselves can be used as carriers. Some of such production processes that have been disclosed are a method in which a polyaluminoxane composition in the form of a solution in an aromatic hydrocarbon solvent such as toluene is brought into contact with a bad or poor solvent and thereby a solid polyaluminoxane composition is precipitated (Patent Literatures 10 and 11), a method in which a solid slurry is obtained by the addition of a salt to polymethylaluminoxane (Patent Literature 12), a method in which a polymethylaluminoxane soluble in a bad or poor solvent is prepared and an organic boroxine is reacted with the polymethylaluminoxane (Patent Literature 13), a method similar to the method described above in which an oxygen-containing compound is reacted with a slurry that contains a solid precipitated by the contact with a bad or poor solvent (Patent Literature 14), and a method in which a polymethylaluminoxane composition in the form of a special solution having a low trimethylaluminum content is heated (Patent Literature 15).
However, the production methods described in Patent Literatures 10 to 14 are problematic from an economic viewpoint in that the polyaluminoxane composition as the solid product is recovered in a low rate relative to the polyaluminoxane composition used as the raw material. Further, these production methods do not specifically consider how to control the particle diameter of the polyaluminoxane composition or do not specifically address the uniformity of particle diameters. Furthermore, Patent Literatures 10 to 15 are substantially silent on the polymer morphology such as bulk specific gravity of olefin polymer particles obtained by the combined use of the solid polyaluminoxane composition with a transition metal compound. In particular, the production method described in Patent Literature 14 cannot produce a polyaluminoxane composition with a uniform particle diameter due to the use of a slurry.
That is, the aforementioned conventional techniques are focused primarily on the superiority over the demerits in the use of solid inorganic carriers and do not substantially reflect the merits in using solid inorganic carriers. For example, the use of silica carriers suppresses the dissolution of polyaluminoxane components into solvents by virtue of the formation of aluminum-oxygen covalent bonds by the reaction of the hydroxyl groups on the silica surface with the polymethylaluminoxane. As a result, the dissolution or the so-called leaching of a cocatalyst component, a main catalyst component or a reaction composition between a main catalyst component and a cocatalyst component into a reaction solvent is prevented from occurring during catalyst preparation steps and/or polymerization (oligomerization) reaction steps. Consequently, olefin polymers having a high bulk specific gravity may be obtained while ensuring excellent operation stability.
When a solid polyaluminoxane composition is used as a cocatalyst carrier in liquid-phase polymerization such as olefin slurry polymerization or a gas-phase polymerization process, it is necessary from the viewpoint of fouling prevention that the occurrence of leaching be suppressed to the minimum. In addition, while polymerization (oligomerization) reaction steps generally involve the addition of highly polar substances such as antistatic agents having a high-polarity functional group such as an ionic functional group or a polyether functional group in the molecule, leaching should be suppressed to a sufficient extent even in the presence of such highly polar substances.
Patent Literature 11 discloses that a solid polymethylaluminoxane composition described in Examples has a solubility in n-hexane of 1.0 mol % or more. Further, Patent Literature 15 discloses that a solid polymethylaluminoxane composition described in Examples has a 12 mol % or less molar fraction of methyl groups derived from trimethylaluminum moieties relative to the total number of moles of the methyl groups and this measurement is feasible by 1H-NMR in tetrahydrofuran-d8 that is a highly polar compound. Namely, it is described that the tetrahydrofuran-d8 soluble components in the solid polymethylaluminoxane composition have a high content of polymethylaluminoxane and a low content of trimethylaluminum.
Patent Literature 15 discloses a method for producing a solid polymethylaluminoxane composition while enhancing the uniformity of particle diameters. However, there is no specific description as to how to control the particle diameter of the solid polymethylaluminoxane composition. According to the disclosure, it is necessary that a polymethylaluminoxane composition in the form of a special solution be used as a raw material in order to obtain a solid polymethylaluminoxane composition with a high recovery rate relative to the amount of the polymethylaluminoxane composition used as the raw material. However, the raw material has a low trimethylaluminum content and this fact causes a problem in the storage stability of the raw material itself (Patent Literature 8). In addition, the production of the special polymethylaluminoxane composition solution entails the use of a high concentration of dangerous trimethylaluminum as a raw material. Thus, severe constrains are imposed on production facilities in order to realize commercial-scale production.
Patent Literature 15 uses a polymethylaluminoxane composition solution prepared by the reaction of trimethylaluminum with an oxygen-containing organic compound. Because of this configuration, the technique is incapable of controlling as desired the particle diameter of the solid polyaluminoxane produced from the polyaluminoxane composition solution. Usually, solid polyaluminoxane compositions used in liquid-phase polymerization processes such as slurry polymerization or gas-phase polymerization processes desirably have a carrier particle diameter that is optimum for the production process, and this is the case particularly when the compositions are applied to existing facilities for such polymerization processes. On the other hand, from an economic viewpoint, it is desired that a solid polyaluminoxane composition be produced with a high recovery rate relative to the amount of a polyaluminoxane composition used as a raw material in view of the expensiveness of the polyaluminoxane composition used as the raw material. However, no method has been reported which can easily produce a solid polyaluminoxane composition having a uniform particle diameter from a commercially available polyaluminoxane composition solution as a raw material while allowing the particle diameter to be varied as desired and also achieving a high recovery rate.